This invention relates to litz wires for high-speed drive coils, low-loss coils and so on, and particularly to a litz wire suitable for easy high-precision assembly and automatic production of coils.
The conventional litz wire, as shown by the cross-section of FIG. 1A, is formed of a plurality of strands 3 each of which is composed of a conductor 1 covered with an insulating layer 2, these strands being twisted together and then covered over the peripheral surface with an adhesive layer 4 so as to have a finished external form 5. Therefore, the strands 3 are fixed only by a frictional force due to the twisting and are not particularly fixed firmly. Consequently, when the litz wire is stressed upon coil assembly or the like, the shape of the litz wire is deformed in the manner shown by the cross-section of the wire illustrated in FIG. 1B. Particularly when the number of the strands 3 constituting the litz wire is increased, the frictional force between the strands due to the twisting is decreased so that its cross-sectional shape is easy to be deformed by an external force.
The deformation of the cross-sectional shape of the litz wire makes the high-precision assembly and automatic production of coils difficult, which fact is a serious problem.
FIG. 2 is an external view of a magnetic bubble memory drive coil produced by using the litz wire. FIGS. 3A and 3B show cross-sectional views taken along line 3--3 in FIG. 2, corresponding to the finished external shapes 5 and 6 shown in FIGS. 1A and 1B. If the cross-sectional shape is deformed from a circular to an elliptical shape as shown by the finished external shape 6, the winding density n in a certain coil length (number of turns per unit length) is caused to decrease greatly. Since the inductance L of a coil is proportional to the square of the winding density n, the inductance L is changed greatly by the deformation of the cross-sectional shape.
Therefore, even though the drive coil is designed satisfactorily as shown in FIG. 3A or the structure of the litz wire is designed well as shown in FIG. 1A, change of the cross-sectional shape of the litz wire at the time of assembly and production of coil as shown in FIGS. 1B and 3B will make the coil specification (inductance L with respect to a constant coil shape and so on) difficult to maintain.
On the other hand, in order to reduce the high frequency loss including D.C. loss in the high-speed drive coil for the magnetic bubble memory, generally the conductor diameter of strands 3 is reduced so that the influence of skin effect is also reduced and the number of strands is increased so that the D.C. loss is reduced. In the past years, the request for high speed drive coils with high frequency is not large so that the conductor diameter of the strands is large and thus the number of strands is small, which results in the cross-sectional shape being little deformed and causes no trouble. However, as the request for high-speed drive coil becomes greater it is absolutely necessary to increase the number of strands and as a result the deformation of the cross-sectional shape becomes inevitably important.